Steering stability and performance of a vehicle are largely characterized by the vehicle's understeer and oversteer behavior. The vehicle is in an understeer condition if the vehicle yaw is less than the operator steering input, where turning the steering wheel more does not correct the understeer condition because the wheels are saturated. The vehicle is in an oversteer condition if the vehicle yaw is greater than the operator steering input. Surfaces such as wet or uneven pavement, ice, snow or gravel also present vehicle stability and handling challenges to the operator. Similarly, in a panic or emergency situation, such as during obstacle avoidance, an operator may react by applying too much steering or failing to counter-steer to bring the vehicle back to its intended path. In any of these cases, the actual vehicle steering path deviates from the intended steering path.
Vehicle stability controls have progressed from first generation systems based upon braking and traction control (braking and powertrain torque management) technologies to more recent systems including independent and coordinated controls of brake, powertrain, steering and suspension damping sub-systems. Typically, distributed control modules are employed to directly interface with respective actuators to effect the desired sub-system controls.
Semi-active suspension systems are incorporated into some modern vehicles and are generally characterized by dampers that are controlled to change the suspension characteristics of the vehicle based on road conditions, vehicle speed, yaw rate and other considerations. Variable fluid-based dampers are known having discrete damping states and continuously variable damping states which affect both jounce and rebound response of the suspension system. Variability in damping may be attained by variable orifice devices or controlled viscosity fluids (e.g., magnetorheological (MR) or electrorheological (ER)) within the damping device. Variable dampers are used predominantly to achieve low speed ride comfort and high speed handling enhancement (ride and handling). However, variable damping techniques are known to enhance vehicle stability in certain understeer and oversteer situations and may be implemented as part of an overall vehicle stability control.
There is a need to coordinate the interaction between the vehicle stability control system and the suspension damper control sub-system in a way that is efficient, yet clear that the vehicle stability control system has ultimate authority when necessary.